SPDT is an acronym for Single Pole, Double Throw.
Single pole means there is only one switch inside, as opposed to a
double throw switch which acts like two switches side-by-side connected
by the same knob.
Double Throw means that the switch can direct the electricity to go
out from the switch through two different wires, depending on which way
the switch is positioned.
Note that on the switch there are 3 pins. The electricity goes in
the middle pin, and if the switch knob is in one position, the
electricity will flow out the pin on the left, and if the knob is moved
to the other position, the electricity will instead go out the other
It is like a diverter valve for water. Note that if one side of the
SPDT is not connected to anything, it simply acts like an on-off
On an electronic device, even if you want the switch to act like an
on-off switch, it is best for the off postion to actually connect to
ground. If it weren't connected to ground, it is "floating" and can
act like a tiny antenna and pick up voltages from radio transmissions
and other things.
Of course, another way to find out what it is like, is to take the
switch apart. But be sure to get a spare because you may not be able
to get it back together.